BOD refers to the amounts of dissolved oxygen (DO) consumption when some substances especially organic substances in water that can be biologically decomposed by microorganisms. For example, the BOD is called five-day BOD (BOD5) if biological oxidation is performed for five days. BOD is an important index for analyzing the content of organic pollutants in water, and is one of the most important parameters for conventional monitoring of water quality, whose higher values mean more organic pollutants in the water and more serious pollution.
At present, the method of diluting and inoculating is commonly adopted internationally for testing the BOD, which is also called BOD5 method. However this method has the disadvantages of time consuming, intensive work load, complex operations, various interference factors, and poor repeatability. Also, it can not reflect the change of water quality timely, and does not allow the on-line testing of water quality.
For the purpose of overcoming the above disadvantages, a variety of methods for testing BOD have been developed in the prior art. One example is microbial sensor method, which comprises the steps of cultivating the target microorganisms, centrifuging the cultured microorganisms, quantifying and physically adsorbing the microorganisms on the surface of a cellulose membrane or dialysis membrane and the like to provide a biological membrane (biofilm), or chemically embedding the microorganisms by using sol-gel polymers to provide a biofilm; and closely attaching the obtained biofilm to the surface of an oxygen electrode, when a target water sample flows through the surface of the biofilm, microbial respiration is enhanced, oxygen consumption increases, therefore the oxygen content tested by the oxygen electrode is reduced. In the method, BOD of a water sample is obtained by use of the principle that microbial respiration is proportional to the content of organic matters. One example is a mediator method, which adopts an artificial electron acceptor, such as potassium ferricyanide to substitute natural electron acceptor oxygen, and thus improves the degradation efficiency of organic matters by microorganisms, and improves the accuracy of the testing results. One example is a biofilm reactor method, which utilizes the enriched microbial population to degrade organic matters, and achieves the measurement on the BOD in water by testing the change of DO content. In order to maintain a stable microbial population, this biofilm reactor method needs continuous supplementation of organic matters, so that the operation is complex and the maintenance cost is high. In prior art, a microbial fuel cell method is also used for testing BOD, which mainly depends on the generation of a current when the enriched microbial population degrade organic matters, and determines the value of BOD in water according to the intensity of the current.
The above methods for testing the BOD in water are all based on the action of microorganisms on organic matters, including microbial respiration and the degradation of the organic matters by the microorganisms. In order to maintain the physiological activity of the microorganisms, a certain level of pH value and osmotic pressure are provided therefore. In the prior art, a buffer solution system is provided for the survival of the microorganisms, and the most commonly used ones of which include phosphate buffer solution and Tris-HCl buffer system. However, the phosphate buffer solution will result in secondary pollution on the environment and Tris-HCl buffer system is expensive and not suitable for on-line monitoring. In the prior art, there is no method for testing the BOD in water that does not adopt a buffer system.